Information recording and/or reproducing apparatus, information recording and/or reproducing method, and phase-change recording medium for use in the apparatus and the methods

ABSTRACT

An information recording apparatus for recording multi-leveled information in a phase-change recording medium by the application of a laser beam thereto by modifying a power level of the laser beam into two or more power levels so as to correspond to the multi-leveled information to be recorded, and setting a plurality of recording mark units including therein at least one recording mark to be formed, based on the modified power levels, so as to correspond to the multi-leveled information, an information reproducing apparatus for reproducing multi-leveled information recorded in the phase-change recording medium with the application of a laser beam thereto, and an information recording and reproducing apparatus, and an information recording method, an information reproducing method, and a phase-change recording medium for use in the above apparatus are disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an information recording apparatus foroptically recording multi-leveled information with high density in aphase-change recording medium comprising a recording layer whichcomprises a phase-change recording material.

The present invention also relates to an information reproducingapparatus for optically reproducing multi-leveled information recordedin the phase-change recording medium.

The present invention also relates to an information recording andreproducing apparatus for conducting the above-mentioned recording andreproduction.

The present invention also relates to an information recording methodand an information reproducing method which can be respectively used inthe information recording apparatus and in the information reproducingapparatus.

The present invention also relates to a phase-change recording mediumfor use in the information recording apparatus, the informationreproducing apparatus, the information recording and reproducingapparatus, the information recording method, and the image reproducingmethod.

2. Discussion of Background

As one of optical recording media which are capable of recordinginformation therein and reproducing recorded information therefrom withthe application of a laser beam thereto, there is conventionally known arewritable recording medium comprising a recording layer which comprisesa phase-change recording material. The phase-change recording materialused in the recording layer of the rewritable recording medium iscapable of performing reversible phase changes between an amorphousphase and a crystalline phase, so that the rewritable recording mediumhas such excellent advantages over conventional recording media that (1)recording and erasing of information can be carried out, using a simpleoptical system and that (2) new information can be easily recorded inthe recording layer, while previously recorded information is beingerased therefrom at the same time.

Generally, in such a phase-change recording medium as mentioned above,an amorphous portion in the recording layer is used as a recordedportion, while a crystalline portion in the recording layer is used asan erased or non-recorded portion.

The recording of information is actually carried out by applying a laserbeam having a power on a recording power level to a recording layer in acrystalline state of the recording medium to heat the laser-beam appliedportions of the recording layer to the melting point thereof or to atemperature above the melting point thereof, and then rapidly coolingthe heated portions to form recording marks in an amorphous state in therecording layer so as to correspond to the information to be recorded.

The recorded information can be erased by the steps of (1) applying alaser beam having a power on an erasing power level to the recordinglayer of the recording medium to elevate the temperature of therecording layer to a crystallization temperature thereof, and (2) thengradually cooling the recording layer, thereby crystallizing therecording marks in the amorphous state in the recording layer.

The recording marks are in the amorphous state and the backgroundthereof is in the crystalline state. The recording marks in theamorphous state and the background in the crystalline state have adifferent reflectivity or a different phase in the light reflectedtherefrom. By use of such differences between the recording marks andthe background thereof in the recording medium, the changes in thequantity of the light reflected by the recording medium can be detected,and recorded information can be reproduced by detecting the changes inthe quantity of the light reflected by the recording medium. Therecording medium may be in the form of a disk.

In the above-mentioned information recording method using the phasechanges in the recording layer, in order to attain high recordingdensity, it is necessary to reduce the size of the recording marks.However, there is a limit to the reduction in the size of the recordingmark.

Furthermore, in the phase-change recording by forming recording marks inthe above-mentioned heat mode, the shape of the recording marks is aptto be significantly affected by the variation in the amount of light ofthe laser beam. When the shape of the recording marks is varied, as amatter of course, the recording of information and the reproducing ofrecorded information cannot be carried out precisely in a stable manner.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide aninformation recording apparatus which is capable of recordingmulti-leveled information in a phase-change recording medium preciselyin a stable manner.

A second object of the present invention is to provide an informationreproducing apparatus which is capable of reproducing multi-leveledinformation recorded in a phase-change recording medium precisely in astable manner.

A third object of the present invention is to provide an informationrecording and reproducing apparatus which is capable of recordingmulti-leveled information in a phase-change recording medium, andreproducing multi-leveled information recorded in a phase-changerecording medium, both precisely in a stable manner.

A fourth object of the present invention is to provide an informationrecording method for recording multi-leveled information in aphase-change recording medium precisely in a stable manner.

A fifth object of the present invention is to provide an informationreproducing method for reproducing multi-leveled information recorded ina phase-change recording medium precisely in a stable manner.

A sixth object of the present invention is to provide an informationrecording and reproducing method for recording multi-leveled informationin a phase-change recording medium and reproducing multi-leveledinformation recorded in a phase-change recording medium, both preciselyin a stable manner.

A seventh object of the present invention is to provide a phase-changerecording medium for use in the above-mentioned information recordingapparatus, information reproducing apparatus, information recording andreproducing apparatus, information recording method, informationreproducing method, and information recording and reproducing method.

A first object of the present invention can be achieved by aninformation recording apparatus for recording multi-leveled informationin a phase-change recording medium by the application of a laser beamthereto, comprising:

-   -   power level modification means for modifying a power level of        the laser beam into two or more power levels so as to correspond        to the multi-leveled information, and setting a plurality of        recording mark units including therein at least one recording        mark to be formed, based on the modified power levels, so as to        correspond to the multi-leveled information.

In the above information recording apparatus, it is preferable that eachof the recording mark units have a power level that corresponds to thetotal area of the one or more recording marks included in each of therecording mark units.

Further, in the above information recording apparatus, it is alsopreferable that each of the recording mark units include one recordingmark and have a track-direction length in a range of 0.5 to 1.0 times abeam diameter defined by 1/e², and that the area of the recording markin each of the recording mark units be changed for recording themulti-leveled information.

The second object of the present invention can be achieved by aninformation reproducing apparatus for reproducing multi-leveledinformation recorded in a phase-change recording medium in the form ofrecording marks by the application of a recording laser beam thereto, bythe application of a reproducing laser beam thereto, comprising:

-   -   reproducing means for reproducing the recording marks based on        reference clock signals, with the timing of detecting a mark        edge of each of the recording marks and the timing of detecting        the intensity of a reflection light from each of the recording        marks being made different.

In the above information reproducing apparatus, it is preferable thatthe reproducing laser beam have a smaller beam diameter than a beamdiameter of the recording laser beam in terms of a beam diameter definedby 1/e².

The third object of the present invention can be achieved by aninformation recording and reproducing apparatus for recordingmulti-leveled information in a phase-change recording medium by theapplication of a recording laser beam thereto, reproducing multi-leveledinformation recorded in a phase-change recording medium by theapplication of a reproducing laser beam thereto, comprising:

-   -   power level modification means for modifying a power level of        the recording laser beam into two or more power levels so as to        correspond to the multi-leveled information, and setting a        plurality of recording mark units including therein at least one        recording mark to be formed, based on the modified power levels,        so as to correspond to the multi-leveled information, and    -   reproducing means for reproducing the recording marks based on        reference clock signals, with the timing of detecting a mark        edge of each of the recording marks and the timing of detecting        a reflection light intensity of each of the recording marks        being made different.

In the above information recording and reproducing apparatus, it ispreferable that each of the recording mark units have a power level thatcorresponds to the total area of the one or more recording marksincluded in each of the recording mark units.

Further, in the above information recording and reproducing apparatus,it is also preferable that each of the recording mark units include onerecording mark and have a track-direction length in a range of 0.5 to1.0 times a beam diameter defined by 1/e², and that the area of therecording mark in each of the recording mark units be changed forrecording the information.

Further, in the information recording and reproducing apparatus, it isalso preferable that the reproducing laser beam have a smaller beamdiameter than a beam diameter of the recording laser beam in terms of abeam diameter defined by 1/e².

The fourth object of the present invention can be achieved by aninformation recording method for recording multi-leveled information ina phase-change recording medium by the application of a laser beamthereto, comprising the steps of:

-   -   modifying a power level of the laser beam into two or more power        levels so as to correspond to the multi-leveled information, and    -   setting a plurality of recording mark units including therein at        least one recording mark to be formed, based on the modified        power levels, so as to correspond to the multi-leveled        information.

In the above information recording method, it is preferable that each ofthe recording mark units have a power level that corresponds to thetotal area of the one or more recording marks included in each of therecording mark units.

In the above information recording method, it is also preferable thateach of the recording mark units include one recording mark and have atrack-direction length in a range of 0.5 to 1.0 times a beam diameterdefined by 1/e², and that the area of the recording mark in each of therecording mark units be changed for recording the multi-leveledinformation.

In the above information recording method, it is preferable that inmodifying the power level of the laser beam, at least one of the powerlevels be further changed in a power level retention time thereof inaccordance with the multi-leveled information.

In the above information recording method, in modifying the power levelof the laser beam, it is also preferable that the power level of thelaser beam be modified into three power levels, namely, a recordingpower level, an erasing power level and a bias power level, with therespective power levels thereof being set in a decreasing order of therecording power level, the erasing power level, and the bias power level(the recording power level>the erasing power level>the bias powerlevel).

Furthermore, in the above information recording method, it is preferablethat in modifying the power level of the laser beam, at least one of therecording power level or the bias power level be further changed in apower level retention time thereof in accordance with the multi-leveledinformation.

The fifth object of the present invention can be achieved by aninformation reproducing method for reproducing multi-leveled informationrecorded in a phase-change recording medium in the form of recordingmarks by the application of a recording laser beam thereto, by theapplication of a reproducing laser beam thereto, comprising the step:

-   -   reproducing the recording marks based on reference clock        signals, with the timing of detecting a mark edge of each of the        recording marks and the timing of detecting a reflection light        intensity of each of the recording marks being made different.

In the above-mentioned information reproducing method, it is preferablethat the reproducing laser beam have a smaller beam diameter than a beamdiameter of the recording laser beam in terms of a beam diameter definedby 1/e².

The sixth object of the present invention can be achieved by aninformation recording and reproducing method for recording multi-leveledinformation in a phase-change recording medium by the application of arecording laser beam thereto, and reproducing multi-leveled informationrecorded in a phase-change recording medium by the application of areproducing laser beam thereto, comprising steps:

-   -   modifying a power level of the recording laser beam into two or        more power levels so as to correspond to the multi-leveled        information,    -   setting a plurality of recording mark units including therein at        least one recording mark to be formed, based on the modified        power levels, so as to correspond to the multi-leveled        information, and    -   reproducing the recording marks based on reference clock        signals, with the timing of detecting a mark edge of each of the        recording marks and the timing of detecting a reflection light        intensity of each of the recording marks being made different.

In the above information recording and reproducing method, it ispreferable that each of the recording mark units have a power level thatcorresponds to the total area of the one or more recording marksincluded in each of the recording mark units.

In the above information recording and reproducing method, it is alsopreferable that each of the recording mark units include one recordingmark and have a track-direction length in a range of 0.5 to 1.0 times abeam diameter defined by 1/e², and that the area of the recording markin each of the recording mark units be changed for recording theinformation.

In the above information recording and reproducing method, it is alsopreferable that the reproducing laser beam have a smaller beam diameterthan a beam diameter of the recording laser beam in terms of a beamdiameter defined by 1/e².

The seventh object of the present invention can be achieved by aphase-change recording medium comprising a recording layer in whichmulti-leveled information can be recorded by an information recordingmethod for recording multi-leveled information in a phase-changerecording medium by the application of a laser beam thereto, comprisingthe steps of:

-   -   modifying a power level of the laser beam into two or more power        levels so as to correspond to the multi-leveled information, and        setting a plurality of recording mark units including therein at        least one recording mark to be formed, based on the modified        power levels, so as to correspond to the multi-leveled        information.

In the above phase-change recording medium, it is preferable that therecording layer comprise Sb and Te with a Sb/Te content ratio of 2 to 5in terms of atomic %, and at least one element selected from the groupconsisting of Ag, In, Ge, Ga, B, Si, and Al.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a block diagram of an information recording and reproducingsystem of an example of an information recording and reproducingapparatus of the present invention.

FIG. 2 is a schematic illustration of a shape of a recording mark formedin a phase-change recording medium for use in the present invention.

FIG. 3 is a diagram in explanation of a recording laser pulsemodification method of the present invention, and the shapes ofrecording marks formed by the recording laser pulse modification method.

FIG. 4 is a diagram in explanation of an example of an informationrecording and reproducing method of the present invention, in which (a)shows the shape of a laser beam spot and recording marks formed in aphase-change recording medium, and (b) shows a recording pulse strategyemployed.

FIG. 5 is a schematic sectional view of an example of a phase-changerecording medium of the present invention, particularly showing alayered structure thereof.

FIG. 6 is a diagram in explanation of the recording conditions in anexample of the present invention, and the results of the measurement ofthe track-direction lengths of recording marks formed.

FIG. 7 is a diagram in explanation of another example of an informationrecording and reproducing method of the present invention, in which (a)shows the shapes of recording marks formed in a phase-change recordingmedium, (b) shows a recording pulse strategy employed in the example,and (c) shows a diagram in explanation of a reproducing waveform.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be explained in detail with reference toFIG. 1.

FIG. 1 shows a block diagram of an information recording and reproducingsystem of an information recording and reproducing apparatus, which isan example of an information recording and reproducing apparatus of thepresent invention.

As shown in FIG. 1, the information recording and reproducing apparatuscomprises a disk 10 which is a phase-change recording medium, arecording system 20 for recording multi-leveled information in the disk10, a reproducing system 30 for reproducing recorded multi-leveledinformation from the disk 10, a beam-diameter setting unit 40 forsetting the diameter of a laser beam for recording multi-leveledinformation and also for reproducing recorded multi-leveled information,and a disk drive unit 50 for driving the disk 10 in rotation. Thedetails of the disk 10 will be explained later.

The recording system 20 comprises a laser beam generation unit 21 forgenerating a laser beam, a power level modification unit 22 formodifying a power level of the laser beam into two or more power levels,and a multi-leveled information setting unit 23 for setting therecording system 20 so as to be capable of recording multi-leveledinformation with 16 levels by modifying the total area of recordingmarks.

The reproducing system 30 comprises a signal detection unit 31 fordetecting signals output from the disk 10 through the beam-diametersetting unit 40 which will be explained in detail later, a timingdetection unit 32 for determining a detection period of signals based onreference clock signals, and a signal processing unit 33 for judging thelevel of a reflection light from recording marks recorded in the disk10, based on the signals from the timing detection unit 32.

The beam-diameter setting unit 40 sets a beam diameter of a laser beamfor recording, which is hereinafter referred to as “recording laserbeam”, and a beam diameter of a laser beam for reproducing, which ishereinafter referred to as “reproducing laser beam”, in terms of a beamdiameter defined by 1/e², for instance, in such a manner that the beamdiameter of the reproducing laser beam is smaller than the beam diameterof the recording laser beam.

The disk drive unit 50 drives the disk 10 in rotation and feeds the dataof the number of revolutions of the disk 10 back to the timing detectionunit 32. In the disk 10, there are formed prepits, and the timingdetection unit 32 generates reference clock signals from (1) prepitsignals detected by the signal detection unit 31 and (2) the number ofrevolutions of the disk 10 fed to the timing detection unit 32.

FIG. 2 is a schematic illustration of a shape of a recording mark 101formed in a phase-change recording medium for use in the presentinvention.

As shown in FIG. 2, the recording mark 101 has such a shape that isunsymmetrical with respect to a scanning advancing direction 102 of thelaser beam used in the present invention. A leading edge of therecording mark 101, directed to the scanning advancing direction 102 ofthe laser beam, is hereinafter referred to as leading edge 103, and arear edge of the recording mark 101, opposite to the leading edge 103,is hereinafter referred to as rear edge 104 of the recording mark 101.

The leading edge 103 can be formed so as to have a sharp edge, withexcellent reproducibility, since the shape of the leading edge 103, interms of the curvature thereof, can be determined in accordance with thediameter of the laser beam employed, and the leading edges 103 of therecording marks can be uniformly formed with respect to the positionsthereof within the same recording medium.

In contrast, the rear edge 104 can be easily changed with respect to theshape and the position thereof by changing the power of the laser beamapplied to the recording medium, since in the area near the rear edge104, the state thereof is changed from a rapidly cooled state to agradually cooled state. In other words, the shape and the position ofthe rear edge 104 can be easily controlled by changing the power levelof the laser beam applied thereto.

In the present invention, by utilizing such properties of the rear edge104, the recording of multi-leveled information can be easily carriedout in a stable manner.

Specifically, in the present invention, the power level of the laserbeam employed is modified into two or more power levels so as tocorrespond to the multi-leveled information to be recorded.

With reference to FIG. 3, when a modification strategy using a recordingpower (Pw), a bias power (Pb) and an erasing power (Pe) is employed, theposition of the rear edge 104 can be controlled, in particular, by theerasing power (Pe), so that the length (L) of the recording mark can becontrolled.

For example, with reference to FIG. 3, when an erasing power (Pe) ismodified into three power levels within a range of 2 mW to 10 mW, with arecording power (Pw) being set in a range of 8 mW to 15 mW on thesurface of a disk, and a bias power (Pb) being set in a range of 0 mW to1 mW, the shape of the recording mark can be modified into three powerlevels of a, b and c in accordance with the levels of the erasing power(Pe). By lowering the level of the erasing power (Pe), the rear end ofthe recording mark can be shifted in the laser beam scanning advancingdirection, so that the shape and the area of the recording mark can bechanged. In the present invention, by changing the area of each of therecording marks, the recording of multi-leveled information can befacilitated.

In the present invention, there can be set a plurality of recording markunits, each recording mark unit including therein at least one recordingmark to be formed, based on the modified power levels, so as tocorrespond to the multi-leveled information to be recorded. Each of therecording mark units is caused to have such a power level thatcorresponds to the total area of one or more recording marks included ineach recording mark unit, whereby multi-leveled information can berecorded by the power level that is modified so as to correspond to thetotal area of one or more recording marks included in each recordingmark unit.

In the information recording apparatus of the present invention, each ofthe above-mentioned recording mark units can be set so as to include onerecording mark, and has a track-direction length in a range of 0.5 to1.0 times a beam diameter defined by 1/e², and the area of the recordingmark in each of the recording mark unit can be changed for recording themulti-leveled information.

More specifically, with reference to FIG. 4, one recording mark M isformed in one recording mark unit U. In the information recording methodof the present invention, in accordance with the multi-leveledinformation to be recorded, the length of the recording mark in thedirection of a track of a disk of a phase-change recording medium, whichis hereinafter referred to as a track-direction mark length, can bechanged multi-stepwise. By changing the track-direction mark length ofthe recording mark, the area of recording mark in each of the recordingmark units U can be changed. In other words, the ratio of the area ofthe recording mark, which is in an amorphous state, to the area of thebackground thereof, which is in a crystalline state, is changed inaccordance with the multi-leveled information to be recorded.

It is preferable that the recording mark unit U have a track-directionlength (L) in a range of 0.5 to 1.0 times a beam diameter defined by1/e², more preferably, in a range of 0.75 to 0.9 times a beam diameterdefined by 1/e², and that the recording marks M be formed with apredetermined constant period of P, corresponding to the track-directionlength (L).

In the above example of the present invention, by changing thetrack-direction mark length of the recording mark in the recording markunit U, the area of the recording mark in the recording mark unit U canbe changed, whereby multi-leveled information can be recorded, and therecorded multi-leveled information can be reproduced by detecting thechanges in the intensity of the light reflected from the recording markin each recording mark unit U.

In order to accurately reproduce the changes in the recording marks,various reproducing methods have been devised. In a conventional markedge detection method, the detection of the edge of the recording markand the detection of the signal intensity of the recording mark arecarried out at the same timing. In contrast to this, in the presentinvention, the recording marks are reproduced based on reference clocksignals, with the timing of detecting the mark edge of each of therecording marks and the timing of detecting the intensity of areflection light from each of the recording marks being made different.

Furthermore, in the present invention, when the recording marks arereproduced based on reference clock signals, a laser beam diameter of areproducing laser beam (hereinafter referred to as a reproducing laserbeam diameter) can be made smaller than a beam diameter of a recordinglaser beam (hereinafter referred to as a recording laser beam diameter)in terms of a beam diameter defined by 1/e², whereby the sensitivity fordetecting the recording marks can be increased.

For example, when recording is carried out, using a red laser beam witha wavelength of 600 nm to 700 nm, reproducing is carried out, using ablue laser beam with a wavelength near 400 nm. Reproduction may becarried out, using a high NA lens, such as SIL (Solid Immersion Lens).Furthermore, the beam diameter may be changed using a single opticalsystem, with a focusing position being changed.

In the above case, as shown in FIG. 3, based on reproducing signals2032-2034, a leading mark edge is synchronized with reference clocksignal 2031 as indicated by 2035. Since the track-direction mark lengthof the recording mark is smaller than the reproducing beam diameter, thepeak position of the intensity of the reflected light is shifted in thelaser beam scanning direction as indicated by 2032, 2033, and 2034. Thesignal intensities are sampled at the timing delayed by one clock signalfrom the leading edge. As a result, reproducing signals can beidentified as being at 4 levels (0, 1, 2, 3).

With reference to FIG. 4, a method of controlling the track-directionrecording mark length so as to be less than a reproducing beam diameterφ of a reproducing laser beam B will now be explained.

To begin with, a power level of a laser beam is modified into two powerlevels, preferably three power levels. A specific recording pulsestrategy of the present invention is shown in (b) in FIG. 4. In thisstrategy, the power level is modified into three power levels, usingrectangular pulses. More specifically, the power level is modified intoa recording power level Pw, an erasing power level Pe and a bias powerlevel Pb, with the respective power levels thereof being set in adecreasing order of the recording power level Pw, the erasing powerlevel Pe, and the bias power level Pb (Pw>Pe>Pb). A recording mark M isformed in each recording mark unit U.

The portion A in (b) in FIG. 4 indicates an example of controlling thetrack-direction recording mark length by changing at least one powerlevel, preferably the erasing power level Pe, in accordance with themulti-leveled information to be recorded. By changing the erasing powerlevel Pe, the timing of the erasing of a rear edge portion Me of therecording mark M can be changed. When the erasing power level Pe islowered to an erasing power level Pe1, the crystallization speed isdecreased and the timing of erasing is delayed, so that thetrack-direction recording mark length of the recording mark M isincreased. When the erasing power level Pe is elevated up to an erasingpower level Pe3, the crystallization speed is increased and the timingof erasing comes early, so that the track-direction recording marklength of the recording mark M is decreased. Therefore, by changing theerasing power level Pe to a plurality of erasing power levels, such asPe1, Pe2 and Pe3, the track-direction recording mark length(corresponding to the area of the recording mark M in the recording markunit U) can be changed. More specifically, it is preferable that theerasing power level Pe be changed relative to the recording power levelPw in such a range that satisfies the relationship of Pe/Pw=0.0 to 0.7.

The portion B in (b) in FIG. 4 indicates another example of controllingthe track-direction recording mark length by changing the retention timeof at least one power level in accordance with the multi-leveledinformation to be recorded. In this example, the recording power levelPw or the bias power level Pb is changed stepwise into a plurality oflevels with respect to the power level retention time thereof (whichcorresponds to the pulse width thereof) Tw or Tb. The portion B in (b)in FIG. 4 indicates a preferable example in which both TW and Tb arechanged, with the ratio of the respective retention times of Tw/Tb beingset in a range of 0.3 to 1.5.

When a recording mark M with a short track-direction recording marklength is formed, the power level retention times, TW and Tb, areshortened, while when a recording mark M with a long track-directionrecording mark length is formed, the power level retention times, TW andTb, are lengthened. By lengthening the power level retention time Tw,the track-direction recording mark length of the recording mark M isincreased, and by lengthening the power level retention time Tb, thecooling time is lengthened, so that the track-direction recording marklength of the recording mark M is further increased, with the rear edgeportion Me of the recording mark M being elongated.

For use in the above-mentioned information recording method, aphase-change recording medium comprising a recording layer with thefollowing composition is preferable in view of the above-mentionedcontrol.

FIG. 5 shows a schematic cross-sectional view of the phase-changerecording medium, particularly showing a layered structure thereof,which is an example of a phase-change recording medium of the presentinvention.

In the phase-change recording medium 1 shown in FIG. 5, reference number4 indicates a recording layer. The recording layer comprises Sb and Tewith a Sb/Te content ratio of 2 to 5 in terms of atomic %, preferablywith a Sb/Te content ratio of 3 to 4 in terms of atomic %, with Sb andTe constituting a matrix material of this recording layer, and at leastone element selected from the group consisting of Ag, In, Ge, Ga, B, Si,and Al, which serves as an additive element to the matrix material. Morespecifically, examples of preferable compositions of the recording layerare AgInSbTe, GeAgInSbTe, GeInSbTe, GeGaSbTe, and GaSbTe.

The recording layer with the above-mentioned composition exhibits theproperty that the crystallization speed thereof strongly depends on thepower of the laser beam applied thereto. The crystallization speed ofthe recording layer is so high that the phase-change follow-upproperties in response to recording pulses are excellent.

Other features of this invention will become apparent in the course ofthe following description of exemplary embodiments, which are given forillustration of the invention and are not intended to be limitingthereof.

EXAMPLE 1

A phase-change recording medium comprising (1) a substrate made ofpolycarbonate and (2) a phase-change recording layer formed thereon,with a track pitch of 0.74 μm, which is identical to that of DVD(Digital Versatile Disc), was prepared.

In this phase-change recording medium, recording marks were formed asfollows:

With reference to FIG. 3, a laser pulse modification method 201 wasconducted. The shapes of recording marks formed by the laser pulsemodification method 201 are shown by 202.

The wavelength of the laser beam employed was 635 nm, and the numericalaperture of an objective lens employed was 0.6.

The recording power (Pw) was set at 13 mW, and the bias power (Pb) wasset at 0.2 mW. The erasing power (Pe) was modified into three powerlevels, 7.8 mW (2011), 6.5 mW (2012), and 4 mW (2013).

Depending upon the erasing power level, the shape of the recording markwas changed stepwise to 3 kinds. Specifically, lengths La, Lb, and Lc ofthe recording marks in the laser beam scanning direction wererespectively, La=0.1 μm, Lb=0.15 μm, and Lc=0.2 μm.

Thus, by lowering the erasing power level, the rear edge portion of therecording mark was elongated, and the shape of the recording mark waschanged as shown in FIG. 3.

EXAMPLE 2

The recording marks formed in the phase-change recording material inExample 1 were reproduced by the reproducing method 203 shown in FIG. 3.

The wavelength of a reproducing laser beam employed was 410 nm, and thenumerical aperture of an objective lens employed was 0.7. The power ofthe reproducing laser beam was 0.6 mW.

In FIG. 3, reference numeral 2031 indicates reference clock signals, andreference numerals 2032 to 2034 respectively indicate reproductionsignals of a, b, and c. Based on the reproduction signals, the referenceclock signals are synchronized with the leading edges of the recordingmarks (2035). In order to show this state, in FIG. 3, each reproducingsignal corresponding to the leading edge of each recording mark isdepicted so as to overlap. Since the track-direction recording marklength is smaller than the diameter of the reproducing laser beam, thepeak positions of the signals are shifted in the laser beam scanningdirection in the order of a, b, and c, corresponding to the area of eachrecording mark. As shown in FIG. 3, the intensity of the signal issampled at such a timing that is delayed from the leading edge by oneclock signal. As a result, the reproducing signals can be identified at4 levels, that is, 0, 1, 2, and 3 (levels 0 to 3). By detecting theintensity of the signal at the position shown in FIG. 3, a largedisplacement value can be obtained, and therefore the accuracy of theidentification of the recording mark is increased.

EXAMPLE 3

As shown in FIG. 5, there was prepared a phase-change recording mediumwith a layered structure, comprising a polycarbonate substrate 2, aZnS—SiO₂ layer 3, a recording layer 4, a ZnS—SiO₂ layer 5, an Ag layer6, and an overcoat layer 7, which were overlaid in this order. Therecording layer 4 was composed of Ge (3 atomic %), Ga (7 atomic %), Sb(65 atomic %), and Te (25 atomic %). The track pitch was 0.4 μm. Thenumerical aperture NA of an optical system employed was 0.65. Thewavelength of a laser beam employed was 405 nm.

FIG. 6 shows the recording conditions in this example and thetrack-direction recording mark lengths measured under the recordingconditions. The track-direction length of the recording mark unit was0.5 μm. The track-direction length of the recording mark M recorded withthis period was measured.

In the graph in FIG. 6, the track-direction length of the recording markM is plotted as ordinate and the sum of the level retention time Tw andthe level retention time Tb, that is, Tw+Tb, as abscissa, with the levelretention time Tw and the level retention time Tb being changed, andwith the ratio of Tw/Tb being kept constant at 0.7 (Tw/Tb=0.7). Thetrack-direction length of the recording mark M on the ordinate wasdetermined by measuring the distance between the leading edge and therear edge of the recording mark M at the center of the track.

By increasing the power level ratio of Pe/Pw, the track-direction lengthof the recording mark M was shortened. Furthermore, by shortening thepower level retention time Tw, Tb, the track-direction length of therecording mark M was also shortened.

State F, State E, and State D shown in FIG. 6 indicate the changes inthe track-direction length of the recording mark M in the case where thepower level retention time, Tw+Tb, was fixed and the power level ratioof Pe/Pw was changed from 0.4 to 0.6. State C and State B shown in FIG.6 indicate the changes in the case where the power level ratio of Pe/Pwwas fixed at 0.6 (Pe/Pw=0.6) and the power retention time, Tw+Tb, waschanged.

Thus, the track-direction length of the recording mark M can be changedto 5 levels in the range of 0.4 μm to 0.1 μm. Within a recording markunit with a track-direction length of 0.5 μm, 6 levels of theintensities of reflection light can be formed when there is includedState A where no recording mark M is recorded.

FIG. 7 shows a diagram in explanation of recording signals andreproducing waveforms. (a) of FIG. 7 shows the changes in the shape (orthe track-direction length) of the recording mark M in a recording markunit having a track-direction length of 0.5 μm. Each of States A to Fcorresponded to the conditions shown in FIG. 6, and the recording markswere recorded in accordance with a recording pulse strategy shown in (b)of FIG. 7. (c) in FIG. 7 shows a waveform reproduced from thephase-change recording medium 1 in which the above-mentioned recordingmarks M were recorded. The changes in the intensity of the reflectionlight are on ordinate in (c) of FIG. 7. The intensity of the reflectionlight changed stepwise due to the interference between the recordingmarks. In accordance with the changes in the area of the recording markwhich is in an amorphous state, the levels of the reflection lightintensity changes.

When recording is made in the recording layer with the above-mentionedcomposition by use of the simple recording pulse strategy as shown in(b) of FIG. 7, the reflection light intensity can be changed to 6levels. In other words, 6-leveled information can be recorded andreproduced.

Japanese Patent Application No. 2000-212512 filed Jul. 13, 2000,Japanese Patent Application No. 2001-053390 filed Feb. 28, 2001, andJapanese Patent Application No. 2001-086813 filed Mar. 26, 2001 arehereby incorporated by reference.

1-21. (canceled)
 22. A phase-change recording medium comprising arecording layer in which multi-leveled information can be recorded by aninformation recording method for recording multi-leveled information ina phase-change recording medium by the application of a laser beamthereto, comprising: modifying a power level of said laser beam into twoor more power levels so as to correspond to said multi-leveledinformation; applying at least two laser beams having different powerlevels to form at least one recording mark; varying a power level of oneof the at least two laser beams to set a plurality of recording markunits including therein the at least one recording mark to be formed,based on said modified power levels, so as to correspond to saidmulti-leveled information.
 23. The phase-change recording medium asclaimed in claim 22, wherein said recording layer comprises Sb and Tewith a Sb/Te content ratio of 2 to 5 in terms of atomic %, and at leastone element selected from the group consisting of Ag, In, Ge, Ga, B, Si,and Al.